The present invention relates to a method for producing a molding compound for the manufacture of plastic parts with high resistance to wear, particularly for wear members of conveyor systems and machines, such as pulp-drainage parts in paper machines, in which high molecular low-pressure polyethylene is mixed with a cadmium salt and titanium dioxide and hot-compressed.
In one known method of this type (German Pat. No. 19 25 408), the high-molecular low-pressure polyethylene is mixed in the form of a screened granulate with additives and compressed to form a molding. This is done in a fusion-compression process under relatively low pressure. After cooling, this molding is ground and mixed with 10 to 40% of pure low-pressure polyethylene granulate. This mixture which consists in part of a reclaimed material and in part of pure starting plastic is then again hot-pressed.
After the plasticizing and cooling, there is obtained a semi-finished plastic product which is not only hard and thus resistant to rubbing or frictional wear but is also hydrophilic, so that, upon the action of water, a permanent film of water can remain on the surface of this plastic product. The hydrophilic properties are obtained by the admixed cadmium salt while the hardness is obtained by the admixed titanium dioxide. Such semi-finished plastic products are particularly well-suited for the manufacture of highly stressed plastic parts of machine-wire tables for paper machines over which the water-entraining wires are moved at high speed. Although, in particular, the addition of the cadmium salts considerably improves the sliding properties of such plastic parts of high-molecular low-pressure polyethylene, these parts are still subject to a very large amount of wear.
It is known, it is true, that the wear of such plastic parts can be decreased further by embedding micro-beads in the plastic material. In one known method of this type for the manufacture of polyethylene semi-finished products (French Patent 23 19 662), the starting material, namely the high-molecular polyethylene, in an amount of 79.3% by weight, has added to it a proportion of 16.7% by weight of antistatic acetylene black. The addition of acetylene is intended to improve the sliding properties while at the same time the hardness of the glass micro-beads is to increase the resistance of the plastic parts to wear. For this reason also the proportion of glass micro-beads, namely 16.7% by weight, is relatively high. The practical results show, however, that with a low-pressure polyethylene prepared in this manner it is not possible for several reasons to obtain the desired wear properties.
On the one hand, a statistically uniform distribution of the glass micro-beads in the plastic is not obtained so that regions of different density of the embedded glass micro-beads are present in the plastic and accordingly result in different wear properties of these regions. Both, because of this and due to the low cross-linking of the polymer molecule chains the glass micro-beads loosen from the plastic when the plastic parts in question are subjected to wear. Furthermore, as a result of the glass beads which protrude from partially eroded plastic, the parts which slide over the plastic guide, such as for instance the screen wires of paper machines, are subjected to excessively great stress.